Audio frequency amplifier



May 12, 1942. E. s; LUND IE AUDI'O FREQUENCY [AMPLIFIER Fiied Nov. 30, 1939 Patented Mey'12,'1942 UNITED STATE PATENT OFFICE 4 2,282,870.

AUDIO FREQUENCY AMPIJFIER" Ernest S. Lundie, Haddon Heights, N. J., assignor' to Radio-Corporation of America, a corporation of Delaware Application November 30,1939, Serial No. 306,955

1 Claim." (oi. 179-111 The present invention relates to a. f. amplifiers, and has for its object to provide animproved a. f. amplifier circuit and 'means therein whereby the, amplifier gain and frequency characteristic may be controlled effectively.

It is also an object of the present invention to provide'an'a. f. amplifier having an improved tone control circuit embodying inverse feedback without resorting to variable loading means for the signal conveying circuits as is the case with the usual'type of tone control system.

It is a still further object of the present in-- vention to provide an improved control system for an a. f. amplifier stage whereby the frequency characteristic of an amplifier may be varied at the high and low frequency-ends thereof through a single control circuit in connection withsaid stage without, interaction with volume control means in the same stage. v i

The invention will, however, be better understood from the following description when. considered in connection with the accompanying drawing, andits scope is pointed out inthe appended claim.

In the drawing,

Figure 1 is a schematic circuit diagram of an' auf. amplifier embodying the invention,

Figure 2 is a similar schematic circuit diagram of a portion of the amplifier shown in Fig. 1 illustrating a modification of the invention, and

Figure 3 is a graph showingthe curves illustrative of certain operatingcharacteristics of the circuits of Figs. 1 and 2.

'- Referring to Fig. 1, 5 is a firststage a. f. am-

plifier tube provided with a signal input circuit 6-1 and an output circuit 8-9 couped to a volume control potentiometer 0 through a coupling capacitor II. i

Thevolume control contact i2 is connected through a limiting resistor l3 and a lead l to the controlgrid l5 of a second stageamplifier tube IS. The lead I4 is also coupled through a limiting r sistor I1 and a lead l8 to a second volume contro potentiometer i9 having input terminals,-

one of which is common with the input terminal I and the other of which is indicated at 20.

with this arrangement, dual volume control is provided on the stage l6. Signals may be received on thegrid 15 from input terminals 6 1 0 through the input stage 5 or directly from input terminals 1-20. By adjusting the-potentiometer devices I2 and I9, signal mixing from thetwo signal sources may be effected. a A self-bias resistor 2| in circuit with the cathode 22 of the said tube It "provides biasing potential forithe grid i5. With an amplifier tube of the type 6J'lat IS, the self-bias resistor 2| may be of the order of 1000 ohms. The input circuits and preceding am lifier 5 are representative of ume control means stage amplifier stage l6 which includes suitablevolfor'regulatingor adjusting the strength of the signal applied to the control grid l5.

The input circuit arrangement of the amplifier It further includes a tone control network 23 having a'feedback connection 24 with the output circuit 25 of a succeeding amplifier stage 20. The latter is coupled through suitable means 21 with the output circuit 28 of the amplifier stage It, so that a feedback loop is provided through the stages-I6 and 20. With the feedback connection between the output circuit of the stage 26 and the cathode terminal 29 of the preceding stage It, the feedback is degenerative.

The external portion of the feedbackcircuit includes-a limiting resistor 30, .a'high frequency control path comprising a series resistor 3,| and a shunt capacitor 32 therefor, a low frequency control path comprising a variable resistor having a resistor element 33 and a variable contact 24 together with a shunt bypass capacitor 35.". The resistor 30 and the tone control means or control paths are connected serially in the order'named and as shown, between the output circuit con-- ,nection 24 and the cathode connection 29 on the self-bias resistor 2|. The latter connection includes a switch having a movable contact 36 and two'fi xed contacts. 31 and 38, the contact 31' being included in the feedback circuit and the contact 38 being connected to a relatively largebypass capacitor 99 which is placed in shunt-with the self-bias resistor 2| when the switch is closed' to the contact 38. This capacitor provides the usual low impedance bypass at signal frequencies. In the position shown, the switch is closed to complete the feedback circuit to the'cathode and at the same time replaces the shunt capacitor 39 for the self-bias resistor 2| with a bypass circuit comprising two resistor or impedance. elements 40 and serially connected, the latter being in the form of a potentiometer having'a contact 42 which is connected to the low potential terminal of the resistor 4| through a bypass or tone control capacitor 43.

p This provides a second high frequency control path connected in shunt with first high frequency control path and'the low frequency control path between the feedback point 29 onthe cathode 22 andthe common circuit return path or ground.

The output circuit 25 of theamplifier stage 26 may be coupled to any suitable load device, such as a loudspeaker indicated at 45 through coupling 4 means provided in the present example, by ia transformer. 1 J In one embodiment of the inventiomthe tone control circuit comprises a resistor 30 of substantially'82000ohms, a capacitor at 32 of 820 mi. shunting the resistor 3| of 470,000 ohms, while any suitable input circuit arrangement for the the capacitor 35 has a value ofthe order of .01

mi. and the maximum resistance of the resistor 33 being of the order of 2-4 megohms. In the shunting circuitfor the self-bias resistor 2i, the resistor 40 has a resistance of substantially 100 ohms, while the resistor M is substantially 5000 ohms maximum. The capacitors 39 and 43 may have values of 20 and .35 mf., respectively.

The operation of the system may be considered in connection with the curves shown in Fig. 3, to which attention is now directed along with Fig. 1.

With the switch 36 closed to the contact 38, the inverse feedback circuit is disconnected and the self-bias resistor 2| is shunted by a relatively low impedance path comprising the capacitor 39, rendering operation of the input circuit of the amplifier substantially normal and providing a relatively high gain and uniform output over a relatively wide a. ,f. range, as indicated by the curve 50 in Fig. 3. Thus, a high normal uniform response may be provided by the amplifier by closing the switch to the contact 38.

When tone control or variation of the frequency characteristic of the amplifier is desired,v

the switch is closed to the position shown, thereby removing the bypass capacitor 39 from the circuit and completing the feedback circuit from the output connection 24 to the cathode 22, thus reducing the gain of the amplifier to a lower level, as indicated by the curve l5253 which represents substantially a flat response characteris tic of lower amplitude level.

With the contact 34 moved to the lowermost position, as viewed in the drawing, thereby reducing to substantially zero the resistance in circuit of the resistor section 33, and disregarding the shunting eifect ofthe capacitor 32, the

inverse feedback current from the output connection 24 is limited by the resistor 3! and the series controlling resistor 30 which also prevents loading of the output circuit 25. The amount of feedback applied to the cathode 22 of the driver stage IB is such that substantially uniform gain over the a. f. range is provided. This is assuming also that the contact 412 is adjusted to the lowermost position on the resistor 4| thereby removing from the shunt circuit for the cathode resistor 2| the shunting effect of the capacitor 43.

By connecting the capacitor 32 in shunt with the series control resistor 3|, that is, across a portion of the series resistance comprising the resistors 30 and 3|, additional feedback voltage is provided at the high frequency end of the amplifier range, causing a reduction in the gain of the amplifier in that range and resulting in a response characteristic for the amplifier as indicatedby the portion of the curve shown at 54 in Fig. 3. The impedance of the capacitor 32 Withrespect to the resistance of the resistor section 3| is such that the desired high frequency attenuation is obtained, and the high frequency control paths constituted by the capacitor 32 and resistor 3| may remain fixed thereafter. 1

The predetermined high frequency response established in the path 3 I32 may now be varied by operation of the contact 42 on the potentiometer Al which is at the ground or cathode end of the feedback circuit, to gradually shunt the resistance of the potentiometer by the capacitor 43, thereby permitting the feedback energy at the higher frequencies to be bypassed around the cathode resistor 21 across which the feedback potentials are applied to the input circuit of the stage 16. The capacity of the capacitor 43 is such that, when fully shunted across the potentiometer 4|, the high frequency response characteristic may be restored to a level as shown an upper limit or level for the response characteristic, between which and the lower limit a variable control range is provided. As hereinbefore noted, the variable control is placed at the ground end of the feedback circuit, as is desirable, and permits the control elements to be placed remotely as desired without regard for extensive shielding.

Considering now the low frequency tone control means in the stage IS, the capacity value of the capacitor 35 is such that it offers an appreciable impedance to feedback energy in the low ermost portion of the operating range of the amplifier and is gradually made effective in the circuit or adjusted to a desired effective value, by operation of the contact 34 to vary the value of the resistor 33. As the contact 34 is moved upwardly as viewed in the drawing to increase the value of the resistance 33 in shunt with the capacitor 35, the normal low frequency feedback characteristic 53, as established by the limiting resistors 30 and 3| hereinbefore referred to, is reduced because of the impedance inserted in circuit, resulting in a risein the low frequency response characteristic as indicated by the portion 55 of the curve in Fig. 3, the relative impedance values of the capacitor 35 and resistor 33 in the feedback circuit determining the upper limit to which the curve 55 rises. Thus, by varying the resistor 33, the low frequency response of the amplifier may also be varied between limits such as indicated by the curves 53 and 55, and tone control for the low frequency end as well as the high frequency end is provided in the same circuit without interaction. By applying the tone control to'the cathode circuit, variable gain or volume control at I2 and I9 may be included in the input circuit of the tone controlling amplifier stage.

From the foregoing, it will be seen that high and low frequency tone control feature is provided by inverse feedback as distinguished from the usual loading circuits. The feedback-circuit end of the. attenuation range for high frequencies and the series impedance element in the variable path serving to limit the upper end of the attenuation range for high frequencies.

In certain amplifiers, it is desirable to provide a wider range of control of the high and low frequency ends of the amplifier operating range for tone control purposes. For example, it is desirable to extend the control range between the curves 54 and SI to a wider range including a aasas'zo characteristic indicated by the curve 56, so that, for certain operating conditions, high frequency tone accentuation may be obtained.

Likewise for the low frequencyend of the op- I erating range, it may be desirable to suppress the low frequency response to provide a characteristic as indicated by the curve 51, the full operating range then being between the curves 55 and 51.

A modification of the circuit of Fig. 1 for the purposes above mentioned is shown in Fig. 2 to which, along with Fig. 3, attention, is now. directed and in which like reference numerals are applied to like circuits-and circuit elements as in Fig. 1. The amplifier tube ltis provided withv a cathode resistor 2| having the cathode terminal 29 to which inverse feedback potentials are applied from the output circuit 2515f the second stage amplifier 26 through a suitable connection 26, and an inverse feedback circuit'including the seriscon trolling resistor 3ll--3l, the latter portion of which is shunted by the high frequency controlling capacitor 32. The low fre- However, in the present circuit, the high fre-' quency tone control includes the resistor MA as a variable or adjustable element. This may be adjusted to substantially zero resistance, in which case, as the resistance approaches zero, the shunting effect of the capacitor A3 is rendered more complete across the cathode resistor. 2| as the contact 42 is moved in an upward direction as viewed in the drawing, thereby subwith the cathode side of the capacitor 35, as indicated at 63in a path 86 shunting not only the capacitor 35 but the capacitor 32, as the feedback control is thereby rendered more effective at lower frequencies than that permitted by the impedance of high frequency control path iii-'32.

The reactance of the choke coil 62 is such that the path 64 provides-no appreciable conductive circuit for feedback energy in the higher frequency range or midrange of the frequency characteristic of the amplifier, but is of relatively low impedance to feedback currents in the low frequency end of the operating range of the amplifier. Therefore, as the value of the resistance 60 in circuit is decreased, the low frequency feedback-voltage is further'increased, causing a" re-' duction in the gain or; the amplifier in the low frequency range, as indicated by the curve 58 and permitting the feedback to be further increased to a value such that the amplifier gain a is appreciably attenuated in the low frequency stantially reducing the high frequency feedback to zero and permitting the high frequency response of the amplifier to rise to a higher limit as indicated by the curve 55. Thus the upper limit of the attenuation range of the high frequency variable tone control means may be adjusted by adjusting the value of the series resister element QiJA.

In the present example,- with the circuit elemerits having values as previously referred to,

when the resistor A is adjusted to substantially 100 ohms, the frequency characteristic. of the amplifier at the high frequency end is made substantially fiatas shown by the curve 55! when the resistor 4! is shunted by the capacitor 83.

The effectiveness of the low frequency control capacitor is the same as in the circuit previously described in that it olTers an appreciable impedance to thefeedback energy in the low frequency end of the operating range of the amplifier thereby reducing the feedback and permitting a higher gain in the amplifier'in that,

range, as shown by the curve 55. In order to carry this response characteristic not only to the normal fiat characteristic indicated by the curve portion 53 but also to attenuate the low fre.-'

quency output of the amplifier to extend the low frequency tone control range, the low frequency feedback control path constituted by the capacitor 35 is provided with a variable control rea terminal 6! at the input end of the limiting resister connected to take feedback voltage from range along a response curve indicated at 51.

It will be seen that the low frequency control path constituted by the reactance 62 and the variable resistor 60 in the circuit 64 provides a branch feedback path from the terminal iii to the cathode terminal 29 independently operable to reduce the low frequency response of the amplifier from that indicated at and determined by the value of the capacitor 35 as an upper limit of the attenuation range, and the curve portion 5'? which may be any desired lower limit of the attenuation range.

From the foregoing, it will be seen that the high and low frequency tone control ranges as provided by inverse feedback may be extended t6 provide not only high and lpw frequency attenuation but a rise in the high and-low frequency response characteristic of the amplifier, as may be desired for any particular amplifier. It will viding a feedback connection with said circuit terminating in a preceding amplifier stage, a high frequency control path in said connection including a limiting resistance and shunt capacity for at least a portion thereof efiective to increase the inverse feedback in a higher frequency range, a low frequency control path serially in circuit with said first-named path having arelatively high impedance at a predetermined lower frequency range and including a capacitor, of relatively low impedance with respect to the first-named capacitor and of relatively high im-v pedance in a predetermined low frequency range and a variable shunt resistance element,\a cathode circuit for said preceding amplifier stage having an impedance element serially in circuit with said first and second-named frequency-controlling paths, a capacitor having a relatively low impedance in a frequency range corresponding to the high frequency end of the operating range of said amplifier, and means forvariably connecting said capacitor in shunt with at least a portion of said last-named impedance element. ERNEST S.LUNDIE. 

